Recycling of a wafer comprising a multi-layer structure after taking-off a thin layer

ABSTRACT

The invention relates to a method of transferring useful layers from a donor wafer which includes a multi-layer structure on the surface of the donor wafer that has a thickness sufficient to form multiple useful layers for subsequent detachment. The layers may be formed of materials having sufficiently different properties such that they may be selectively removed. The layers of material may also include sub-layers that can be selectively removed from each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International ApplicationPCT/IB2004/000311 filed Jan. 7, 2004, and claims the benefit ofprovisional application 60/472,435 filed May 22, 2003, the entirecontent of each of which is expressly incorporated herein by referencethereto.

FIELD OF INVENTION

This invention relates to recycling of a donor wafer after taking off alayer of semiconductor material. This recycling includes removal ofmaterial involving a portion of the donor wafer on the surface where theuseful layer was taken off.

BACKGROUND OF THE INVENTION

Before taking off the useful layer, the donor wafer comprises asubstrate and a useful layer that is to be taken off or transferred fromthe substrate. The useful layer is typically obtained by epitaxiallydepositing the layer on the substrate.

After removal, the useful layer is integrated with a structure in whichcomponents will be formed, particularly in the fields ofmicroelectronics, optics, or optoelectronics, for the most part.

The layer to be taken off must have a high level of quality determinedaccording to one or more specific criteria. The quality of the layer tobe taken off largely depends on the growth support, that is, on thequality of the substrate on which it is epitaxially deposited.

The formation of such a high quality substrate is often complex andrequires particular attention, involving technical difficulty and ahigher cost. The cost is further increased when considering the removalof a layer of a composite semiconductor material such as an alloy. Inthis situation, the epitaxy substrate also must exhibit a structurewhich is often difficult and costly to implement. Thus, substrates canbe provided with a buffer layer to specifically avoid such difficultiesof implementation.

The term “buffer layer” refers to a transition layer between a firstcrystalline structure, such as a support substrate, and a secondcrystalline structure. The second structure modifies the structural orstoichiometric properties of the material or a surface atomicrecombination. Buffer layers permit the support structure to include asecond crystalline structure having a lattice parameter that differssubstantially from that of the support substrate.

A first technique of forming a buffer layer includes growing successivelayers so as to form a structure having a composition varying graduallyin thickness, the gradual variation of components of the buffer layerthen being directly associated with a gradual variation of its latticeparameter. A layer, or superposed layers, formed on the buffer layer canbe taken off from the donor wafer, and transferred to a receivingsubstrate so as to form a well-defined structure.

One of the main applications of a transfer of thin layers formed on abuffer layer concerns the formation of layers of elastically stressedsilicon, and especially, in the case where the silicon is stressed intension, because certain properties, such as electron mobility in thematerial, are distinctly improved. Other materials, such as SiGe, canalso be the subject of a substantially analogous taking-off ortransferring procedures.

A transfer of such layers onto a receiving substrate, specifically by amethod termed SMART-CUT® that is known to those of skill in the art,then permits structures to be formed such as SeOI (Semiconductor OnInsulator) structures.

For example, after taking-off an elastically relaxed layer of SiGe, thestructure obtained, including the taken-off useful layer, can then serveas a growth support for silicon which will be placed under tension bythe layer of relaxed SiGe. As an illustration, an example of such amethod is described in the IBM document of L. J. Huang et al.,(“SiGe-On-Insulator prepared by wafer bonding and layer transfer forhigh-performance field-effect transistors”, Applied Physics Letters,26/02/2001, Vol. 78, No. 9) in which a process is given for forming aSi/SGOI structure.

Other applications of growth on a buffer layer are possible,particularly with Group III-V semiconductors. Transistors are commonlyformed in technologies based on GaAs or based on InP. In terms ofelectronic performance, InP has an appreciable advantage over GaAs. Forthe main reasons of cost and feasibility, the chosen technique includestransferring to a receiving substrate a taken-off useful layer of InPobtained by growth on a buffer layer on a support substrate of GaAs.

Certain taking-off methods, such as an “etch-back” type method, includedestruction of the remaining portion of the support substrate and of thebuffer layer during taking-off. In certain other methods of taking-off,the support substrate is recycled, but the buffer layer is lost.

The technique of formation of a buffer layer is complex. Moreover, tominimize its density of crystallographic defects, the thickness of abuffer layer is generally considerable, typically between one andseveral micrometers. The production of such a buffer layer leads to anoften long, difficult, and costly implementation.

A second technique of production of a buffer layer is disclosed in WO00/15885, which has as its main object to elastically relax a layer ofGe that is stressed by a Ge buffer layer. This technique is based onspecific epitaxy conditions, associating the parameters of temperature,time, and chemical composition. The main advantage of this technique isthat it is simpler, shorter, and less costly to perform. The bufferlayer finally obtained is not as thick as a buffer layer formedaccording to the first technique.

A third technique of formation of a buffer layer is disclosed by B.Hollander et al., “Strain relaxation of pseudomorphicSi_(1-x)Ge_(x)/Si(100) heterostructures after hydrogen or helium ionimplantation for virtual substrate fabrication” (in Nuclear andInstruments and Methods in Physics Research B 175-177 (2001) 357-367).It includes relaxing elastic stresses in the layer to be taken off bydeep hydrogen or helium implantation. This third technique can give aresult close to a buffer layer produced according to one of the twoprevious techniques with substantially less demands of implementation.

The method describes a relaxation of a SiGe layer stressed incompression, this layer being formed on a Si substrate. The techniqueused includes implantation of hydrogen or helium ions through thesurface of the stressed layer in the Si substrate to a given depth,generating perturbations in the thickness of Si above the implanted zone(this thickness then forms a buffer layer) and causing, under heattreatment, a certain relaxation of the SiGe layer. This technique seemsto be shorter, easier to practice, and less costly than the firsttechnique of forming a buffer layer.

An advantage of using this technique would be to later integrate thisrelaxed or pseudo-relaxed layer into a structure for the fabrication ofcomponents, particularly for electronics or optoelectronics.

However, similar to the first technique for forming a buffer layer, abuffer layer made according to one of the last two techniques is removedusing known techniques of recycling of the donor wafer after taking-off.Technical difficulties of implementation remain in carrying it out, sothat improvement of the process is needed.

SUMMARY OF THE INVENTION

The invention relates to a method of transferring useful layers from adonor wafer which comprises forming a multi-layer structure on a surfaceof the donor wafer at a thickness sufficient to provide multiple usefullayers for subsequent detachment; detaching one layer of the multi-layerstructure for transfer as a first useful layer while leaving behind aremaining portion of the multi-layer structure; and removing materialfrom the remaining portion of the formed multi-layer structure toprovide a planarized surface for subsequent detachment of an additionaluseful layer. Preferably, the multi-layer structure has sufficientthickness to form at least three useful layers.

The useful layers are advantageously formed of materials havingsufficiently different properties such that one layer may be selectivelyremoved while at least one other layer remains intact. In oneembodiment, the useful layers include pairs of sub-layers, with eachsub-layer having sufficiently different properties from the othersub-layer in the layer such that one sub-layer may be selectivelyremoved while at least one other sub-layer remains intact. A first layerof the multi-layer structure can be formed of a material having alattice parameter different from the lattice parameter of the adjacentlayers, and can be sufficiently thin such that the first layer may beelastically strained to have a lattice parameter similar to the latticeparameter of the adjacent layers, so that the first layer does notdisturb the crystallographic structure of the adjacent layer or layers.

The useful layers can be removed in any one of a variety of ways. Atleast one useful layer is generally removed by chemical etching, such aselectrochemical or photo-electrochemical etching. Additional usefullayers can be removed mechanically by a selective mechanical-chemicalplanarization. If desired, all useful layers can be removedmechanically, such as by polishing, optionally including abrasion orchemical etching. Also, the useful layer can include a doping element tofacilitate selective removal, or the layers can be made of materialshaving different porosities. A convenient selective removal processcomprises deoxidation of an oxide layer.

A buffer structure can be formed on the donor wafer, wherein the bufferstructure includes a support substrate and a buffer layer between thesupport substrate and the multi-layer structure. This buffer layer maycomprise a semiconductor material of constant chemical compositionhaving a lattice misfit with the support substrate, with the bufferlayer having crystallographic defects in order to relax elastic stressesin the multi-layer structure. The buffer layer also may have a latticeparameter progressing substantially in thickness between a latticeparameter of the support substrate and another lattice parametersubstantially different from that of the support substrate. Furthermore,the buffer structure may comprise an additional layer having one or moreof a sufficient thickness to confine defects or a surface latticeparameter that is different from that of the support substrate.Generally, the buffer structure is formed of a binary, ternary,quaternary, or higher degree atomic alloy belonging to atomic alloyGroup IV-V, Group III-V, or Group II-VI.

The donor wafer may comprise one of the following:

-   -   (A) a support substrate of Si and a buffer structure comprising        a buffer layer of SiGe having a Ge concentration which increases        in thickness and an additional layer of SiGe that is relaxed by        the buffer layer;    -   (B) a support substrate of Si and a buffer structure comprising        a buffer layer of SiGe having with a Ge concentration which        increases in thickness between about 0% and about 100% and an        additional layer of SiGe that is relaxed by the buffer layer,        having a Si concentration of about 0%;    -   (C) a relatively thick layer of Si;    -   (D) a support substrate comprising AsGa in contact with the        buffer structure, the buffer structure comprising a buffer layer        having an atomic alloy of ternary or higher degree, belonging to        Group III-V, and having a composition which chosen from among        the possible combinations of (Al, Ga, In), (N, P, As), and at        least two elements chosen from Group III or V, these two latter        elements having a concentration evolving gradually in the        thickness of the buffer layer;    -   (E) the buffer structure of (D) which further comprises, near        its interface with the support substrate, a lattice parameter        that is close to that of InP;    -   (F) a support substrate of sapphire, SiC, or Si, with a buffer        structure comprising a buffer layer of Al_(x)Ga_(1-x)N, with x        varying from 0 to 1 starting from the interface with the support        substrate, optionally including an additional layer of GaN; or    -   (G) a support substrate of sapphire, SiC, or Si, a mask and a        buffer layer of GaN, optionally with another layer of GaN        between the mask and support substrate.

Preferably, the useful layers to be detached comprise Si or elasticallystressed Si; Ge or elastically stressed Gei, SiGe and Ge; AsGa and Ge;an alloy belonging to Group III-V; GaAs, InP; InGaAs; AlN, InN, or GaN.Also, the multi-layer structure preferably comprises one of thefollowing:

-   -   (A) two layers of almost elastically relaxed SiGe, and an        intermediate layer between the SiGe layers comprising: Si        elastically strained to have a lattice parameter similar to the        lattice parameter of the adjacent SiGe layers; SiGe with a Ge        concentration substantially different from the Ge concentrations        in each of the two adjacent layers, and being elastically        strained to have a lattice parameter similar to the lattice        parameter of the adjacent layers; doped Si; or doped SiGe;        wherein at least one of the layers has properties sufficiently        different from the other layers to permit selective chemical        etching of the adjacent layer;    -   (B) two layers of GaAs, and an intermediate layer of AlGaAs        intermediate between the GaAs layers, wherein at least one of        the layers has properties sufficiently different from the other        layers to permit selective chemical etching of the adjacent        layer;    -   (C) two layers of substantially elastically relaxed Si, and an        intermediate layer between the Si layers comprising SiGe        elastically strained to have a lattice parameter similar to the        lattice parameter of the adjacent Si layers, doped Si, or doped        SiGe; wherein at least one of the layers has properties        sufficiently different from the other layers to permit selective        chemical etching of the adjacent layer;    -   (D) two layers of InP, and an intermediate layer of InGaAsP        between the two layers of InP, wherein at least one of the        layers has properties sufficiently different from the other        layers to permit selective chemical etching of the adjacent        layer;    -   (E) an intermediate layer of InN between a AlN layer and a GaN        layer; or    -   (F) an intermediate layer of InN between two layers of GaN.

It is also preferable for the donor wafer to include at least one layerthat contains up to about 5% carbon.

The buffer layer may be formed by epitaxy of a superficial layer on thesupport substrate by stabilizing the support substrate to apredetermined first temperature; chemical vapor phase depositing of abase layer at the said predetermined first temperature to apredetermined thickness that is less than a desired final thickness;increasing the predetermined first temperature to a second predeterminedtemperature; and chemical vapor phase depositing additional material atthe predetermined second temperature until the desired final thicknessof the superficial layer is obtained. The buffer layer is that portionof the superficial layer that interfaces with the support substrate andextends over a thickness for which the rate of crystallographic defectsis greater than a limit value, with the remaining portion of thesuperficial layer representing being at least a portion of themulti-layer structure.

In one embodiment, the multi-layer structure is completely formed byepitaxy on the superficial layer. In yet another embodiment, the bufferlayer is formed by forming an elastically stressed layer on the donorwafer; forming a perturbation zone, capable of forming structuralperturbations, at a given depth; and supplying energy to bring about anat least relative relaxation of at least part of the elasticallystressed layer, with the relative relaxation taking place across aregion delimited by the perturbation zone and by the stressed layer,this region also confining crystallographic defects and representing abuffer layer.

The buffer layer can also be formed by an Epitaxially LaterallyOvergrown (ELOG) technique on a support substrate comprising at thesurface a mask in relief according to specific patterns, the bufferlayer being the thickness of the layer deposited by lateral epitaxy inwhich defects are confined relative to the layers deposited by lateralepitaxy thereon and representing the multi-layer structure. The specificpatterns of the mask may be periodically spaced electrically insulatingbands that are substantially mutually parallel and sufficiently fine tonot perturb the ELOG. In this embodiment, the support substrate maycomprise a solid substrate; an intermediate layer having a latticeparameter close to the nominal lattice parameter of the materialdeposited by ELOG; or the mask.

The buffer layer may be formed so as to have its lattice parameterprogressing substantially in thickness between the lattice parameter ofthe support substrate and another lattice parameter substantiallydifferent from the lattice parameter of the support substrate.Additionally, the buffer structure may further comprise an additionallayer, and the method further comprises, before the formation of themulti-layer structure, forming of an additional layer having asufficient thickness to confine defects or a surface lattice parameterthat is substantially different from that of the substrate. Generally,at least one of the layers of the multiplayer structure is formed bycrystalline growth.

The invention also relates to a recyclable donor wafer comprising asubstrate and a multi-layer structure on the substrate, with themulti-layer structure having sufficient thickness to form (a) at leasttwo useful layers that can be detached, and (b) additional material thatcan be removed to planarize exposed surfaces of the useful layers priorto detachment from the donor wafer, with the useful layers being formedof at least one of SiGe, Si, an alloy belonging to Group III-V, and withthe substrate having a composition chosen from among possiblecombinations of (Al, Ga, In) or (N, P, As). Preferred materials of thesubstrate and the multi-layer structure disposed thereon include thosethat are described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, purposes and advantages of this invention will becomeclearer upon a review the following detailed description and exampleswhich is made with reference to the accompanying drawing figure, inwhich:

FIG. 1 shows the different steps in a process according to the inventionthat includes taking off a thin layer from a donor wafer followed byrecycling of the donor wafer after taking off;

FIG. 2 shows a first donor wafer before taking off according to theinvention;

FIG. 3 shows the various steps in a process according to the inventionincluding successively taking off a thin layer starting from a donorwafer, and recycling the donor wafer after taking off; and

FIG. 4 shows a second donor wafer according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a process of recycling a donor wafer(10) after taking off a useful layer formed of a material chosen fromamong semiconductor materials. The donor wafer (10) includes a substrate(1) and a multi-layer structure (I), the multi-layer structure (I)includes the taken-off useful layer. The process includes removal ofmaterial from a portion of the donor wafer (10) on the side where thetaking-off has occurred, while preserving a portion of the multi-layerstructure (I′). The multi-layer structure is formed to include at leastone other useful layer which can be taken off after recycling, without asupplementary step of reforming the useful layer.

The invention also relates to a process of making a donor wafer thatwill donate a useful layer by taking-off, and that can be recycled aftertaking off using the recycling process. The process includes forming amulti-layer structure on a substrate for subsequent transfer of theuseful layers of the structure.

The invention further relates a method of taking-off a useful layer on adonor wafer to be transferred onto a receiving substrate. The methodincludes the generally steps of:

-   -   (a) bonding the donor wafer with the receiving substrate on the        side of the useful layer to be taken off;    -   (b) detaching the useful layer included in the multi-layer        structure of the donor wafer; and    -   (c) recycling the donor wafer in accordance with the recycling        process.

The method may be performed cyclically by taking off a useful layer froma donor wafer, re-forming a useful layer, and taking off an additionaluseful layer. The useful layers are used in a donor wafer that includesa receiving substrate and the useful layer. The useful layer may beformed of at least one of the following materials: SiGe, Si, an alloybelonging to the atomic alloy Groups III-V, and having a compositionchosen from among possible combinations of (Al, Ga, In)-(N, P, As).

The invention also relates to a donor wafer that donates a useful layerby taking-off, and recycled or capable of being recycled by a recyclingprocess. The donor wafer includes a substrate, a multi-layer structurethat supplied the useful layer, and after taking off, having asufficient thickness to form at least one other useful layer that can betaken off.

The invention relates to recycling a donor wafer. The wafer includes amulti-layer structure, after taking off at least one useful layer (inother words the part of the donor wafer that was taken off) in order tointegrate it into a semiconductor structure. The recycling may beperformed such that the remaining part of the multi-layer structure canonce again supply a useful layer in a subsequent taking off afterrecycling without implementing a step to reform the useful layer, suchas crystalline growth by epitaxy. Therefore, recycling must includeadapted treatment to not deteriorate part of the multi-layer structurein which the useful layer is included, so that this useful layer canstill be taken off after recycling.

In one embodiment, several useful layers that can be taken off may beincluded in the recycled multi-layer structure, so as to have a sequenceof several successive taking off operations between which a recyclingprocess may be applied.

With reference to FIGS. 1 a, 1 b, 2, 2 a, 2 b, 3 and 4, the donor wafer10 includes a substrate 1 and a multi-layer structure I. The substrate 1may be formed of a single crystalline material with the first latticeparameter, or it may be a “pseudo-substrate” that includes a supportsubstrate and a buffer structure interfaced with the multi-layerstructure I. A “buffer structure” refers to any structure behaving likea buffer layer.

Preferably, the structure at the surface has a fairly relaxedcrystallographic structure without a significant number of structuraldefects. The buffer layer performs at least one of the followingfunctions: reduction of the density of defects in the upper layer oradaptation of a lattice parameter between two crystallographicstructures with different lattice parameters.

To perform the second function, the area around the periphery of one ofthe faces of the buffer layer has a first lattice parameter almostidentical to the lattice parameter of the support substrate, and thearea around the periphery of its other face has a second latticeparameter almost identical to the lattice parameter of the layer of themulti-layer structure I directly adjacent to the buffer structure.

In one embodiment, the buffer structure includes a single buffer layer.The buffer layer located on the support substrate has a latticeparameter on its surface significantly different from the latticeparameter of the support substrate, so that it has a layer in the samedonor wafer 10 with a lattice parameter different from the latticeparameter of the support substrate. In some applications, the bufferlayer may prevent the adjacent layer from containing a high density ofdefects or from being significantly strained. In some applications, thebuffer layer may also result in a good surface condition of the adjacentlayer.

According to a first technique for making the buffer structure, a bufferlayer is formed so as to have a lattice parameter that is globally andprogressively modified over a significant thickness, to form thetransition between the two lattice parameters. This type of layer isusually called a metamorphic layer. This modification to the latticeparameter may be made continuously within the thickness of the bufferlayer.

The modification may be done in “stages,” each stage being a thin layerwith an almost constant lattice parameter different from the latticeparameter for the adjacent stage, so as to modify the lattice parameterdiscretely stage by stage. The modification may also have a more complexform, such as a variation of a variable content composition, aninversion of the sign of the variation of the content, or discontinuoussteps in the composition.

The variation of the lattice parameter in the buffer layer isadvantageously found by increasing the concentration in the buffer layerof at least one atomic element that is not included in the supportsubstrate, progressively starting from the support substrate. Forexample, a buffer layer made on a support substrate of a unitarymaterial, or of a binary, ternary, quaternary, or higher degreematerial.

The buffer layer is preferably made by growth on the support substrate,for example by epitaxy, using known techniques such as CVD (ChemicalVapor Deposition) and MBE (Molecular Beam Epitaxy) techniques. Ingeneral, the buffer layer may be made by any other known method in orderto obtain a buffer layer composed of an alloy between different atomicelements. A light finishing step of the surface of the support substratesubjacent to the buffer layer, such as by CMP polishing, may beperformed before the buffer layer is produced.

In another embodiment, the buffer layer made according to the firsttechnique is included in a buffer structure that includes a buffer layer(almost identical to the first buffer layer in the first configuration)and an additional layer. The additional layer may be between the supportsubstrate and the buffer layer, or on the buffer layer. This additionallayer may form a second buffer layer, such as a buffer layer to confinedefects, and thus improve the crystalline quality of the multi-layerstructure I made on the buffer structure. This additional layer is madeof a semi-conducting material, which may preferably have a constantmaterial composition. The composition and thickness chosen for such anadditional layer are particularly important for achieving this property.Structural defects in an epitaxial layer usually reduce gradually withinthe thickness of this layer.

In another embodiment, the additional layer is located on the bufferlayer and has a constant composition of relaxed material to fix thesecond lattice parameter.

The additional layer may be located on the buffer layer and have alattice parameter significantly different from the lattice parameter ofthe support substrate. The additional layer may be formed of a materialrelaxed by the buffer layer. The additional layer is advantageously madeby growth on the buffer layer, such as by epitaxy, CVD, or MBE.

In a first embodiment, the growth of the additional layer is made insitu, directly continuous with the formation of the subjacent bufferlayer, the subjacent buffer layer in this case also advantageously beingformed by layer growth. In a second embodiment, the additional layer isgrown after a surface finishing of the subjacent buffer layer, forexample by CMP polishing, heat treatment, or any other smoothingtechnique.

A second technique for making the buffer structure is based on atechnique for depositing a surface layer on a support substrate, thissurface layer having a nominal lattice parameter significantly differentfrom the lattice parameter of the adjacent material on the surface ofthe support substrate. The surface layer deposit is formed so that thedeposited layer is nearly free from plastic defects such asdislocations. A first part of the surface layer is in contact with thesupport substrate, that confines plastic defects such as dislocationsand a second part of the surface layer is relaxed or pseudo-relaxed bythe first part, and has few or no plastic defects.

The first part of the deposited surface layer acts as a buffer layer byconfining plastic defects so as to preserve the second part of thesurface layer and adapting the lattice parameter of the surface layer tomatch the lattice parameter of the substrate.

“Confinement” means that most plastic defects are located in the firstpart. The second part of the surface layer is not absolutely free ofdefects, but the concentration of defects is compatible withmicroelectronics applications.

The deposition technique used to make such a buffer layer advantageouslyincludes variations of temperatures and chemical compositions of thedeposit with time. Thus, a buffer layer can be made with a chemicalcomposition that is almost constant throughout its thickness, unlike abuffer layer made according to the first technique. One or more layersmay be inserted between the buffer layer and the second part of thesurface layer. The thickness of the buffer layer may be less than theminimum thickness of buffer layers made according to the firsttechnique. WO 00/15885 discloses an example of such a buffer layeraccording to this technique, by depositing SiGe or Ge on amonocrystalline Si support substrate.

For example, this type of deposition process may be used to deposit ofmonocrystalline Ge on a monocrystalline Si support substrate, accordingto the following steps:

-   -   (A) temperature stabilization of the monocrystalline silicon        support substrate at a first predetermined stabilized        temperature of about 400° C. to 500° C., preferably about        430° C. to 460° C.;    -   (B) chemical vapor deposition (CVD) of Ge at the first        determined temperature until a base layer of Ge is obtained on        the support substrate with a predetermined thickness less than a        required final thickness;    -   (C) increase in the temperature of the chemical vapor deposition        of Ge from the first predetermined temperature up to a second        predetermined temperature of about 750° C. to 850° C.,        preferably about 800° C. to 850° C.; and    -   (D) continuation of the chemical vapor deposition of Ge at the        second predetermined temperature until the final required        thickness is obtained for the surface layer of monocrystalline        Ge.

The buffer layer is the part of the deposited layer that interfaces withthe support substrate and extends over a thickness in which thecrystallographic defects ratio is greater than a limiting value. Inparticular, the thickness of this buffer layer may be on the order ofabout 0.5 to 1 micron, which is less than the thickness of a bufferlayer made according to the first technique. The other part of the layeris at least part of the multi-layer structure I. This type of depositionprocess may also be performed by other means, such as that disclosed inWO 00/15885. The result is production of the substrate 1 of the donorwafer 10, the substrate 1 including the support substrate and the bufferlayer.

A third technique for making a buffer structure that is used on thewafer and that includes the substrate 1 and a layer deposited on thesubstrate 1. The material chosen to form this buffer layer generally hasa nominal lattice parameter significantly different from the latticeparameter of the surface of substrate 1, so that it has to beelastically strained in compression or tension by substrate 1.

The general structure of the strained layer is made of a strainedmaterial, but it may also contain one or several thicknesses of relaxedor pseudo-relaxed material for which the accumulated thickness issignificantly less than the thickness of the strained layer, so that thestrained layer remains in a globally strained state. In all cases, thestrained layer is preferably formed on substrate 1 by crystalline growthsuch as by epitaxy, and using known techniques, such as the CVD or MBE.

In order to obtain such a strained layer without too manycrystallographic defects, such as isolated defects, or extensivedefects, such as dislocations, crystalline materials are preferablychosen to form the substrate 1 and the strained layer (close to itsinterface with substrate 1) so that the difference between the first andsecond nominal lattice parameters is sufficiently small. For example,the difference in the lattice parameter is typically about 0.5% to about1.5%, but it could be higher.

For example, in Group IV-IV materials, the nominal lattice parameter ofGe is about 2.4% greater than the lattice parameter of Si, and thereforeSiGe with 30% of Ge has a nominal lattice parameter about 11.5% greaterthan the lattice parameter of Si.

Preferably, the thickness of the strained layer is almost constant, sothat it has nearly constant intrinsic properties to facilitate futurebonding with the receiving substrate (as shown in FIG. 1 b or 2 b). Toprevent relaxation of the strained layer or the appearance of plastictype internal stresses, the thickness of the strained layer must alsoremain less than a critical elastic strain thickness. This criticalelastic strain thickness depends mainly on the material chosen to formthe strained layer and the difference in the lattice parameter with thesubstrate 1. Those skilled in the art will use information known in theart to determine the value of the critical elastic strain thickness ofthe material to be used for the strained layer formed on the materialused for the substrate 1. Therefore, once the strained layer is formed,it has a lattice parameter approximately the same as the latticeparameter of its growth substrate 1, and is then subjected to internalelastic compression or tension strains.

Once the structure is formed, the third technique for making a bufferstructure includes a step for the formation of a perturbation zone orzone of weakness at a given depth in the substrate 1. A perturbationzone is defined as a zone in which internal stresses exist that can formstructural disturbances in surrounding parts. This zone is preferablyformed over most of the surface of the substrate 1 parallel to thesurface of the substrate 1.

One method for forming such a zone of weakness includes implantingatomic species in the substrate 1 at a given determined depth, with adetermined implantation energy and determined proportioning of atomicspecies. In one embodiment, the implanted atomic species include one ormore of hydrogen and helium.

This type of perturbation zone formed by implantation then includesinternal strains, or even crystallographic defects, exerted by theatomic species implanted on the crystalline network adjacent to thedisturbance zone. These internal strains can then createcrystallographic disturbances in the overlying zone.

According to this method, the buffer layer is made while a second stepis being carried out by an energy input to help with the appearance ofdisturbances in the region overlying the perturbation zone, increase theamplitude of these disturbances in this overlying region, and cause anelastic relaxation at least in the strained layer following theappearance of disturbances. The main purpose of such energy input is tocause at least relative relaxation of the elastic strains in thestrained layer in order to form a relaxed strained layer.

The intermediate region within substrate 1 between the disturbance zoneand the strained layer confines dislocation type defects and adapts thelattice parameter of the substrate 1 to the nominal lattice parameter ofthe strained layer. Therefore, in this case this intermediate region maybe considered a buffer layer.

A heat treatment is advantageously used to generate the sufficientenergy input to cause these structural modifications. This heattreatment is advantageously carried out at temperatures significantlylower than a critical temperature above which a significant number ofimplanted atomic species would be degassed.

Thus, local crystallographic disturbances are created from theseinternal strains in the perturbation zone. These disturbances appearmainly in the buffer layer due to minimization of elastic energy in thestrained layer, and they increase in amplitude under the influence ofthe heat treatment.

When these disturbances have become sufficiently large, they act on thestrained layer by relaxing the elastic stresses in the strained layer atleast in relative terms. These relaxed strains are mainly being due tothe mismatch in the lattice parameters for the nominal mesh in thematerial in the strained layer and the material in substrate 1.Relaxation of the strained layer can also be accompanied by theappearance of inelastic type crystalline defects in the thickness of thestrained layer, such as through dislocations.

Suitable treatments, such as a heat treatment, may be applied to reducethe number of these defects. For example, an adapted treatment may beused to increase the density of dislocations to bring it between twolimiting values, the two limiting values defining an interval ofdislocation densities in which at least some of the dislocations canceleach other out.

In all cases, the end result is a relaxed or pseudo-relaxed layer forwhich the nominal lattice parameter is significantly different from thenominal lattice parameter of the growth substrate 1 and which has a lowcontent of dislocations disadvantageous for the formation ofmicroelectronic components in the relaxed strained layer. This relaxedor pseudo-relaxed layer may form at least part of the multi-layerstructure I. For more information, refer to B. Hollander et al., “Strainrelaxation of pseudomorphic Si_(1-x)Ge_(x)/Si(100) heterostructuresafter hydrogen or helium ion implantation for virtual substratefabrication” (Nuclear and Instruments and Methods in Physics Research B175-177 (2001) 357-367). The buffer layer made using this method isincluded in substrate 1 as defined before use of this third method formaking the buffer layer.

A fourth technique for making the buffer structure is based on a supportsubstrate for the buffer structure to be made, for which the surface isin relief and deposition of the component elements of the bufferstructure on the support substrate. Since the surface of the supportsubstrate is not planar, the deposition of the components of the bufferstructure is made anisotropically with growth selectivity effects andlocal coalescence that result in the constructed buffer structure withdetermined properties.

This fourth method of making the buffer structure uses determinedtechniques with parameters set such that the properties obtained in thebuffer layer correspond to crystallographic defect confinementproperties, so that the multi-layer structure I that will be made on thebuffer structure has an intrinsic quality structure.

The choice of the topography of the support substrate is one of theessential factors for obtaining such a result. Preferably, a topographywith patterns will be chosen that is periodically repeated over theentire surface of the support substrate, in order to have a homogeneousinfluence over the entire surface of the wafer. For example, there wouldbe a support substrate with bands at a fixed distance from each other inorder to concentrate dislocations of epitaxied layers close to thebands, and particularly the corners of the bands. The thickness of thelayer in which most of the dislocations are confined then forms thebuffer layer. The bands are preferably formed of an insulating materialformed on a substrate and which forms a mask to materials that will bedeposited subsequently.

Furthermore, an intermediate layer of crystalline materials may beinserted between a solid substrate and the structure in relief, actingas a substrate for growth of the buffer structure. The insulatingstructure in relief is sufficiently thin so that it does not disturbresumed growth of the buffer structure on the growth substrate subjacentto it. This technique is also called ELOG and is applied mainly todeposits of nitride films by MOCVD (MetalOrganic-Chemical-Vapor-Deposition) epitaxy. For example, the article byShuji Nakamura entitled “InGaN/GaN/AlGaN-Based Laser Diodes with anEstimated Lifetime of Longer than 10 000 hours” in the “MaterialsResearch Community” Bulletin, May 1998, volume 23, No. 5, can be used asa basis, which in particular describes growth of GaN on a structure ofSiO₂ bands. Example 9 below will describe a GaN structure made usingthis fourth technique for making the buffer structure by ELOG.

Regardless of the structural configuration in substrate 1 (which may ormay not include a buffer structure), the substrate 1 is formed from acrystalline material at the interface with the multi-layer structure I,with few or no crystallographic defects. At least some of the differentlayers forming the multi-layer structure I are preferably made by growthon the substrate 1, for example by epitaxy, CVD, or MBE.

In one embodiment, at least some of these layers are grown in situ,directly in continuation with the formation of the layers subjacent toeach layer, the subjacent layers in this case also advantageously beingformed by layer growth. In a second embodiment, at least some of theselayers are grown after applying light surface finishes to the layerssubjacent to each layer, such as by CMP polishing, heat treatment, orother smoothing technique. Finally, a multi-layer structure I isobtained that includes layers of crystalline materials.

The multi-layer structure I before recycling has a thickness of at leasttwo useful layers. However, the multi-layer structure I is preferablythicker than two useful layers to compensate for material thicknessesremoved during treatment(s) applied during recycling to correct defectsoften created when the useful layer is taken off. Roughness, thicknessvariations, structural defects, or other types of defects are frequentlyfound on the surface of a donor wafer 10 after taking off, like thatshown in the post-taking off layer 3′ in FIG. 1 c or FIG. 2 c. Forexample, projections or rough parts may appear on the surface to betaken off of the remaining donor water 10, after taking off.

These surface parts in relief that appeared on the surface of themulti-layer structure I depend mainly on the manner of taking off andthe technique used during taking off. For example, one manner of takingoff frequently used in industry includes taking off the useful layerover only part of the donor wafer 10 (which is typically near thecenter) and leaving projections over the surface of the donor wafer 10,rather than over the entire surface of the donor wafer 10. Theseprojections are typically single-piece and located around the peripheryof the surface of the donor wafer 10, all projections then being calleda “taking off ring” in the business. Known taking off techniques, likethose described in more detail below, or the SMART-CUT® techniquementioned above, will sometimes cause surface roughness.

Therefore the thickness of the multi-layer structure I before taking offmust be at least two useful layers to be taken off and a thicknessgreater than or equal to a thickness margin corresponding to the minimumamount of material to be removed during the recycling operation(s). Thisthickness margin is typically of the order of one micron in the case ofrecycling after a SMART-CUT® type of taking off (which is generallyknown and described below). However, this thickness margin can bereduced by using high performance recycling techniques such as selectivechemical etching.

One main type of processing applied during recycling is a process forremoving material such that only part of the multi-layer structure Iremains, including at least one other useful layer that can be taken offafter recycling. This material removal is applied on the donor wafer 10,at the free surface of the multi-layer structure I that remains aftertaking off.

In one particular recycling process, a surface treatment is applied toremove part of the multi-layer structure I on which the useful layer wastaken off. A surface thickness containing surface defects that appearedduring taking off is removed, such as dislocation type crystallographicdisturbances or isolated defects.

Several surface treatments such as the following can thus be appliedindividually or in combination:

-   -   (A) dry or wet chemical etching, preferably applied in a bath,        by plasma, or by atomization; etching may be chemical,        electrochemical, or photo-electrochemical alone, or any other        equivalent etching, such as etching applied during        mechanical-chemical polishing;    -   (B) annealing, for example under hydrogen;    -   (C) chemical etching, for example under HCl accompanied by        annealing; or    -   (D) surface oxidation, carried out using oxidation techniques        known to one skilled in the art, on the surface of the        multi-layer structure I, followed by removal of the oxidized        layer by deoxidation, preferably using a chemical process (such        as attack by a hydrofluoric acid bath) preferably preceded by an        annealing step. This type of sacrificial oxidation process is        described below.

A surface treatment may also significantly improve the surface conditionof the multi-layer structure I remaining after taking off, and also theuniformity of its thickness, particularly in the case of the last foursurface treatments. This is particularly useful if bonding is appliedwhen the useful layer is taken off. Regardless of the surfacetreatment(s) selected, the surface quality of the multi-layer structureI may be improved compared with the surface condition before thistreatment was applied.

In a first case, the improvement in the structural and geometric qualityof the multi-layer structure I is sufficiently good so that a usefullayer can be taken off without any additional material removaltreatments. In this case, and in a first configuration of themulti-layer structure I before taking off, the multi-layer structure isformed of several layers of the same material with almost the sameproperties. In this case and in a second configuration of themulti-layer structure I before taking off, this multi-layer structureincludes several layers, such as shown in FIGS. 1 a, 2 a, 3 or 4, withmaterial properties significantly different from each other at each oftheir interfaces. Regardless of the configuration of the multi-layerstructure I, after taking off, material is removed from the surface overa given thickness so that finally there will be one or more layersremaining including one or more useful layers that can be taken off in asubsequent taking off operation.

In a second case, the improvement in the structural and geometricquality of the multi-layer structure I obtained after use of the surfacetreatment is not sufficient for a useful layer to be taken off withoutsubsequent material removal treatments. The additional treatmentspreferably include selective removal of one layer with regard to theadjacent layer, the selectivity between the two layers being dueessentially to significant differences in the properties of the twomaterials forming these two layers.

Selective material removal may also follow another material removaldifferent from that obtained after a surface treatment. For example, amore massive removal of material on part of the multi-layer structure Imay have been applied, such as mechanical removal by grinding,polishing, abrasion, or bombardment. However, a treatment for selectivematerial removal may also be applied without necessarily applying priortreatments such as surface treatments or more massive material removals.

In order to apply such a selective material removal, before taking off,the multi-layer structure I includes a layer that stops removal of thematerial in the overlying layer. The two materials that make up the twolayers close to their interface are chosen such that there is a means ofselectively removing material, with a capacity for attacking the layerto be removed significantly greater than the capacity for attacking thestop layer. Taking off then applies to the part of the multi-layerstructure I above the stop layer, on the side of the stop layer oppositethe substrate 1.

Several techniques for the selective removal of material may be used forthe protection layer 3. A first technique for the selective removal ofmaterial includes exerting friction forces on the layer to beselectively removed, to pull off at least part of the material to beremoved. For example, these friction forces may be exerted by apolishing plate, possibly combined with an abrasive or chemical action.

The material that forms the stop layer is chosen from semiconductors sothat there is a mechanical means of attacking the material with asignificantly lower capacity to mechanically attack the material formingthe stop layer than the capacity to attack the material in the overlyinglayer to be removed, and thus being suitable for the use of at least oneselective mechanical attack. The material in the stop layer then hassignificantly greater resistance to the mechanical attack used than thelayer overlying it.

-   -   Consequently, it would be possible to suitably harden the        protection layer 3 to make it more resistant than the overlying        layer to the mechanical attack selected to remove the overlying        layer. For example, a semiconductor material such as Si,        carbonated with a concentration of C typically between about 5%        and 50%, is harder than the same non-carbonated semiconductor.    -   A second technique for selective removal of material includes        chemically and selectively etching the material to be removed. A        wet etching process may be used with etching solutions adapted        to the materials to be removed. A dry etching process may also        be used to remove material, such as etching by plasma or        atomization. Etching may also be chemical, electrochemical, or        photo-electrochemical.    -   The material from which the stop layer is formed is chosen from        among semiconductors so that there is a fluid (a gas or a        solution depending on whether the etching is dry or wet) for        etching with a significantly lower capacity to etch the material        forming the stop layer than the material in the overlying layer        to be removed, and thus being capable of implementing selective        etching.    -   However, the term “stop layer” has a primary function of        “stopping” etching, even if there is no absolute stopping of        etching. This is particularly the case for etching by        atomization, which is more accurately referred to as        “atomization rate” or “attack rate”.

In general, the selectivity of etching a layer A with respect to a layerB is quantified by a selectivity factor related to the ratio$\frac{{Etching}\quad{rate}\quad{for}\quad{layer}\quad A}{{Etching}\quad{rate}\quad{for}\quad{layer}\quad B}$

The stop layer thus acts as a barrier to chemical attack, by protectingitself and the layer subjacent to it (which includes substrate 1). Theselectivity of chemical etching between the material in the stop layerand the material in the layer to be removed by selective etching may beobtained by the two materials being different; the two materialscontaining almost identical atomic elements, except for at least oneatomic element; the two materials being almost identical, but the atomicconcentration of at least one atomic element in a material issignificantly different from the atomic concentration of the same atomicelement in the other material; or the two materials having differentporosity densities.

For example, SiGe behaves like a stop layer with regard to etching of Siwith a solution containing compounds such as KOH (selectivity about1:100), NH₄OH (selectivity about 1:100) or TMAH (tetramethyl ammoniumhydroxide). SiGe with a concentration of germanium greater than or equalto 25% behaves like a stop layer with regard to etching of SiGe with agermanium concentration less than or equal to 20%, with a solutioncontaining compounds such as TMAH. Si suitably doped with a selecteddoping element and a selected concentration, such as boron at more than2×10¹⁹ CM⁻³, behaves like a stop layer for etching undoped Si with asolution containing compounds such as EDP (ethylene diamine andpyrocathechol), KOH, or N₂H₂ (hydrazine). Porous Si is etched using anetching that is selective with regard to non-porous crystalline Si,using a solution containing compounds such as KOH or HF+H₂O₂.

This chemical removal of material may also be accompanied by the use ofmechanical or other means for attacking the material. In particular, CMPpolishing may be used with a selective chemical etching solution. Aselective chemical etching may also be preceded or followed by removalof material made by mechanical means of attacking the material such aspolishing, grinding, abrasion or any other means.

A third technique for selective removal of material includes applyingsacrificial oxidation. In this respect, the multi-layer structure Iincludes an oxidizable layer with a greater oxidation capacity than thesubjacent layer, and that will be the layer to be removed selectivelywith respect to the subjacent layer to be kept that acts as the stoplayer. The oxidizable layer is included within the multi-layer structureI before taking off (and therefore before recycling), and the taking offthen applies to the part of the multi-layer structure I located abovethe oxidizable layer on the side of the stop layer opposite thesubstrate 1.

The oxidizable layer is oxidized after taking off at the surface of themulti-layer structure I at the time of recycling, and then correspondsto a treatment technique for the removal of surface material describedabove. Regardless of the selected configuration, the sacrificialoxidation process includes a step for forming of an oxide layer,possibly an annealing step, and a deoxidation step. Oxidation may applyto the oxidation of one or several layers. Oxidation may also be donenear the surface of the stop layer. Oxidation is done using any of theknown oxidation techniques, such as thermal oxidation.

In the case where thermal oxidation is used, the main parameters are theoxidation temperature and duration. Other important parameters are theoxidizing nature of the atmosphere, the oxygen content, and treatmentpressure conditions. These parameters may be well controlled tofacilitate reproducibility.

The annealing step cures defects that might occur during the oxidationstep. The deoxidation step includes selectively removing the oxide layerwith respect to the stop layer, for example by a chemical process usinga hydrofluoric acid bath, and forms the selective material removal. Forexample, oxidized silicon dipped in a 10% or 20% hydrofluoric acid bathfor a few minutes can typically remove one hundred to several hundrednanometers of thickness of this oxide.

Regardless of the method(s) used for selective removal of material inthe process according to the invention, in all cases they can maintain aquality of the layers of the multi-layer structure I remaining afterrecycling almost identical to the quality that they had beforerecycling, and similar to the original quality during their formation(before the first taking off) such as a layer with a quality similar tothe quality of the epitaxied layer. The removable layer(s) remainingpresent in the multi-layer structure I after recycling according to theinvention thus have very good quality, particularly a structuralquality. Examples of selective removals of material applied duringrecycling, taking off processes that can be used, and donor wafersbefore recycling are detailed below.

With reference to FIG. 1 a, the multi-layer structure I before takingoff is includes a first layer 2 and a second layer 3 on the first layer2, the first layer 2 forming a stop layer for selective removal of thesecond layer 3. Layer 2 and layer 3 are each at least as thick as thethickness of the useful layer.

A method for taking off a thin layer is shown in FIGS. 1 b and 1 c. Afirst preferred take off step according to the invention includescreating a weakening area in the second layer 3, in order to perform adetachment and thus take off the required layer(s).

Several techniques may be used to create a weakening area of this type.A first technique, called the SMART-CUT® known to those skilled in theart (and is described in a number of books dealing with techniques forreducing wafers) includes a first step in which atomic species (such ashydrogen ions) are implanted with a determined energy to thus create aweakening area. A second technique includes forming a weakened interfaceby creation of at least one porous layer, such as described EP-A-0 849788. The weakened area is created between the first layer 2 and thesecond layer 3 or in the second layer 3.

With reference to FIG. 1 b, a second step of taking off a thin layerincludes adding on a receiving substrate 5 to the surface of the donorwafer 10. The receiving substrate 5 forms a sufficiently rigidmechanical support to support the second layer 3, part of which will betaken off the donor wafer 10 to protect it from any mechanical stressesfrom the outside. This receiving substrate 5 may be made of silicon orquartz or any other type of material. The receiving substrate 5 is addedby bringing it into intimate contact with the multi-layer structure Iand bonding it, in which there is preferably a molecular bonding betweenthe substrate 5 and structure 1. This bonding technique, and variationsof it, are described in “Semiconductor Wafer Bonding” (Science andtechnology, Interscience Technology) by Q. Y. Tong, U. Gosele and Wiley.

If necessary, bonding is accompanied by an appropriate treatment of thecorresponding surfaces to be bonded by adding thermal energy or byadding an additional binder. Thus, a heat treatment used during orimmediately after bonding makes the bond rigid. Bonding may also becontrolled by a bonding layer such as silica, inserted between themulti-layer structure I and the receiving substrate 5, with particularlystrong molecular bonding capacities.

Advantageously, the material forming the bonding face of the receivingsubstrate 5 or the material of the bonding layer formed if any, iselectrically insulating to make an SeOI structure from the taken offlayers. The semiconductor layer of the SeOI structure in this case isthe taken off part of the second transferred layer 3. Once the receivingsubstrate 5 is bonded, part of the donor wafer 10 is removed at theweakened area formed previously, by detaching it.

In the case of the SMART-CUT® technique, a second step is performedwherein the area implanted (forming the weakening area) is subjected toa thermal and/or mechanical treatment, or a treatment with any othertype of energy input, to detach it at the weakening area. In the case ofthe second technique, the weakened area is mechanically treated, oranother type of energy is input, in order to detach it at the weakenedlayer. Detachment at the weakening area according to one of these twotechniques removes the largest part of the wafer 10, to obtain astructure from whatever is left of the second layer 3 that was taken off(which in this case represents the useful layer), the bonding layer, ifany, and the receiving substrate 5.

A finishing step on the surface of the structure formed at the taken offlayer is preferably applied to remove any surface roughness,non-homogeneities in the thickness, or undesirable layers, by using, forexample, a chemical mechanical polishing CMP, etching, or at least oneheat treatment. In one embodiment, a stop layer with a selective removalof material may be included in the useful layer in order to improve thefinish of the useful layer by selective removal of material stopped atthis stop layer.

A post-taking off layer 3′ forms the part of the second layer 3 thatremained after taking off and located above the first layer 2 and thecomplete wafer forming a donor wafer 10′ to be sent to recycling so thatit can be reused later during taking off a subsequent layer. The resultof recycling such a donor wafer 10′ is shown in FIG. 1 d. It usesselective removal of material in the post-taking off layer 3′ withrespect to the first layer 2, possibly followed by or preceded by asurface finishing step. The donor wafer 10″ is then capable of providinga useful layer taken off in the first layer 2 during a subsequent takingoff without any additional step.

In another embodiment, the donor wafer 10 includes several pairs, eachformed of a first layer 2 and second layer 3, the second layer of eachpair can be removed selectively with respect to the first layer in thesame pair by a means of selectively removing the material. These layersmay also be referred to as sub-layers. Preferably, one of the layers 2of a pair may be selectively removed with respect to the layer subjacentto it, which is included within the multi-layer structure I. Thisembodiment has the advantage that the layer 3 may be removed selectivelywith respect to the subjacent layer 2 or the layer 2 may be removedselectively with respect to the layer subjacent to it. Still, in onespecial case, the layer subjacent to layer 2 is a layer 3 that belongsto another pair of layers.

One example of a particular configuration of a multi-layer structure Ithat includes several such pairs of layers is given with reference toFIG. 3, in which the multi-layer structure I is composed of a first pairconsisting of a first layer 2A and a second layer 3A and a second pairconsisting of a first layer 2B and a second layer 3B, each layer havingthe same thickness or a greater thickness than the useful layer. In thisembodiment of a donor wafer 10, one or more layers may be removed in oneor more steps, according to one process of the invention. One or moreintermediate recycling steps may be applied by selective removal ofmaterial on the part of the layer remaining after taking off and thesubjacent layer that includes at least one useful layer that can betaken off. Thus, with this particular configuration, it is possible totake off useful layers with one or multiple layers of materials.

With reference to FIG. 2 a, the multi-layer structure I includes a firstlayer 2 inserted between a second layer 3B overlying it and a thirdlayer 3A adjacent to the substrate 1, before taking off. The thicknessof the second layer 3B and the third layer 3A are greater than or equalto the thickness of the useful layer. The material forming the firstlayer 2 is chosen such that there is at least one method of removingmaterial with a capacity to attack the material forming the first layer2 significantly different from the capacity to attack the material in atleast one of the two layers 3A and 3B, at their corresponding interfacesto permit selective removal.

In one embodiment, the thickness of the first layer 2 is at least asgreat as the thickness of a useful layer to be taken off. The result isthen a configuration similar to one of those described above. In asecond embodiment, the crystallographic structure of layers 3A and 3B,and particularly the lattice parameters, are almost identical, and it isundesirable for the first layer 2 to significantly disturb thecrystallographic structure of its adjacent layers 3A and 3B. Inparticular, it would be undesirable to disturb the crystalline growth ofthe second overlying layer 3B during its formation on the first layer 2,for which the lattice parameter must be approximately the same as thelattice parameter for the part 3A subjacent to the first layer 2.

In another embodiment, the first layer 2 is elastically constrained sothat its lattice parameter is almost identical to the lattice parameterof layer 3A subjacent to it, even if the lattice parameter of thematerial in this layer is significantly different from the latticeparameter of the first layer 2. The nominal parameter of the first layer2 and the lattice parameter of the zone subjacent to it (included withinthe third layer 3A) are similar to each other, to prevent the appearanceof defects (such as dislocations or local stresses) in the first layer2. The first layer 2 must be sufficiently thin to prevent progressiverelaxation of the elastic strain through the thickness of the layer orthe generation of defects. To achieve this, the thickness of such afirst layer 2 formed of an elastically strained semiconductor materialmust be less than a critical thickness known to one skilled in the art,and depending particularly on its component materials, of the materialsin the layers adjacent to it, and techniques for making the strainedlayer.

Critical thicknesses typically encountered for a Si layer 2 between twolayers 3A and 3B of SiGe (50%-50%) are approximately a few tens ofnanometers.

In a second embodiment of the first layer 2, a material is chosen forthe first layer 2 with a nominal lattice parameter nearly the same asthe lattice parameter of the materials from which the zones adjacent toit are made. Thus, unlike the first embodiment, the crystallographicstructure of the first layer 2 in this case is significantly relaxed.

Consequently, and also to satisfy a selectivity criterion during removalof material applied during the first recycling step, the material forthe first layer 2 will be chosen such that it has at least oneconstituent element that is not in at least one of the materialsadjacent to it. The material in the first layer 2, however, has alattice parameter similar to the lattice parameter of adjacent zones,this constituent element is then the essential element that willdetermine selectivity with respect to the adjacent layer consideredduring selective removal of material. In one case, no constituentelement of the material in the first layer 2 is present in the materialmaking up the adjacent zone concerned by selective removal of material,and the two materials are then entirely different. In another case,every constituent element in the first layer 2 that is different fromthe adjacent zone concerned by selective removal of material may be anadditional element or an element that does not exist in the adjacentlayer being considered.

For example, a first layer 2 with approximately the same latticeparameter as adjacent zones could be doped, so that this latticeparameter is not significantly disturbed after doping. If the firstlayer 2 is formed of the same material as the material from which theadjacent zone by the selective removal of material is made, this dopingelement is the element that will determine the selectivity capacity.When doping the first layer 2, the thickness of the first layer 2 may insome cases remain less than a critical thickness known to those skilledin the art, if defects such as dislocations, and particularly throughdislocations, are not wanted.

In a third embodiment of the first layer 2, the layer 3A made previouslyis made porous on the surface in order to make a porous layer. Thisincrease in porosity may be applied by anodization or by any otherporosity technique, such as that described in EP0 849 788 A2.

This layer of porous material may make a first layer 2, if at least oneadjacent material can be selectively attacked using an appropriateattack method. The porosity does not significantly disturb thecrystallographic structure of these two adjacent layers, such that afirst layer 2 does not significantly disturb the crystallographicstructure of the donor wafer 10. The result is a crystallographicstructure of the first layer 2 that is very similar or identical to thecrystallographic structure of the zones adjacent to it, such that thefirst layer 2 does not disturb the crystallography of the surroundingstructure.

Alternatively, there may be a first layer 2 with a specific influence onthe lattice parameter of the surrounding structures, the state (strainedor relaxed, complete or relative) that the first layer 2 may induce inadjacent layers, in these cases represents a property considered to havevery little advantage for the following application.

With reference to FIGS. 2 b and 2 c, part of the layer 3B is taken offand transferred onto a receiving substrate 5 according to a taking offmethod similar to those described above with reference to FIGS. 1 b and1 c. After taking off and during recycling, selective removal ofmaterial applied at the first layer 2 includes at least one of thefollowing selective removals of material: as shown in FIG. 2 d,selective removal of the material in the layer 3B′ adjacent to the firstlayer 2, the first layer 2 forming a stop layer for removal of material;after removal of layer 3B′, selective removal of the material in thefirst layer 2, the material in the third layer 3A adjacent to the firstlayer 2 forming a stop layer for removal of material.

In one particular method of selective removal, two successive selectiveremovals of materials may be combined at the first layer 2. In thiscase, the second layer 3B, and then the first layer 2 are selectivelyremoved.

Regardless of the method chosen for selective removal of material in thefirst recycling step and the method chosen to remove the part of thedonor wafer 10 on the same side as the useful layer, there is a stoplayer for removal of material (the first layer 2 in the case of thefirst selective removal of material or the zone subjacent to the firstlayer 2 included in the third layer 3A in the case of the secondselective removal of material). The stop layer acts as a barrier formaterial attack, and in particular protects the third layer 3A in whichthere is a new useful layer to be taken off during a later taking off.

In embodiment, the donor wafer 10 includes several triplets formed ofthe first layer 2, second layer 3B, and the third layer 3A. The secondlayer 3B or the first layer 2 of each triplet may be removed selectivelywith respect to the layer subjacent to them, and form part of the sametriplet, by removing the material selectively. Preferably, there is alsoa method of removing material in a third layer 3B of a tripletselectively with respect to a layer subjacent to it that is alsoincluded in the multi-layer structure I. This embodiment has theadvantage that any layer in a triplet of layers can be selectivelyremoved during recycling.

In this embodiment, the layer subjacent to the third layer 3B is asecond layer 3A belonging to another triplet of layers. Alternatively,the layer subjacent to the third layer 3B is a first layer 2 that doesnot belong to the same triplet as layer 3B. In this case, the firstlayer 2 in the triplet overlying it and to which the third layer 3Bbelongs is preferably inserted with another triplet subjacent to it. Inthis case, the result is an assembly formed from a sequence of type 3Alayers and type 3B layers separated by a type 2 layer. An example of aparticular configuration of a multi-layer structure I that several suchtriplets of layers is shown in FIG. 4, in which the multi-layerstructure I includes a first triplet formed of a layer 2A insertedbetween a layer 3A and 3B as shown in FIG. 2 a; a second triplet formedof a first layer 2C inserted between a layer 3C and 3D as shown in FIG.2 a; and a layer 2B inserted between the two triplets. The thickness ofeach of the layers 3A, 3B, 3C and 3D is at least the thickness of theuseful layer to be taken off. The first function of the layers 2A, 2Band 2C is to protect the layer subjacent to them from removal ofmaterial applied during recycling by forming a stop layer to selectiveremoval of the layer overlying it during recycling; or by forming alayer to be removed selectively with respect to the layer subjacent toit. Therefore, in this configuration of a donor wafer 10, there arelayers 3A, 3B, 3C, and 3D in which useful layers can be taken off,separated by protection layers 2A, 2B, and 2C respectively protectingthe layers subjacent to them during recycling.

Therefore, according to one process of the invention, one or more layersmay be taken off in one or more steps, with one or more intermediaterecycling operations by selective removal of the part of the layerremaining after taking off with respect to the first protection layerencountered (2A, 2B or 2C), this layer thus protecting a layer subjacentto it and that includes at least one useful layer that can be taken off.Thus, with this configuration it is possible to take off useful layersformed by one or more sub-layers of materials.

When several layers are taken off, at least one protection layer or partof a protection layer such as layers 2A, 2B and 2C are also taken off.This layer may act as a protection layer when selectively removingmaterial when the surface of the taken off useful layer is beingfinished rather than during recycling, particularly to remove surfaceroughness.

In general, and for a multi-layer structure I according to the inventionin which several useful layers can be taken off between the recyclingsteps, a cyclic process for taking off useful layers from a donor wafer10 is used by performing the following steps iteratively in turn: ataking off process and a recycling process. Before the cyclic taking offmethod is applied, a process for making the donor wafer 10 may beimplemented with one or several techniques for making thin layers on thesubstrate I described above.

The taking off may be repeated several times starting from a donor wafer10 (such as one of those shown in FIGS. 1 a, 2 a, 3, or 4) in the samestructure I made on substrate 1 without necessarily forming additionallayers or without necessarily applying a treatment to retrieve at leastpart of the substrate 1. This saves time with implementation of theglobal taking off process, and makes implementation of the processeasier and significantly less expensive than the different processesaccording to the state of the art.

The number of taking off operations performed on the multi-layerstructure is a function of the thickness of the multi-layer structure.Eventually, the remaining multi-layer structure I is no longersufficiently thick to contain a useful layer to be taken off. In a firstcase, the donor wafer 10 is thrown away and in this case the entiresubstrate 1 is lost. This makes the substrate may be complex, longer,and more expensive, particularly if the substrate 1 includes a bufferstructure.

In another embodiment, at least part of the substrate 1 is recoveredusing a recycling process. If the substrate 1 includes a bufferstructure, three types of recycling of the substrate 1 may be applied. Arecycling process that includes removal of the entire buffer structure,but in which it is possible to keep at least part of the supportsubstrate on which the buffer structure was formed, may be used. Thisrecycling causes loss of part of the substrate 1, however, which isusually the most difficult and most expensive to make and requires theuse of an additional step to form a buffer structure. For example, itmay be required to reform a substrate 1 equivalent to the substratebefore recycling.

A recycling process that includes removal of part of the bufferstructure, and is capable of keeping the support substrate on which thebuffer structure was formed and part of the buffer structure, such as abuffer layer that is expensive to make, may be used. For example, whenrecycling is finished, selective removal of material is possiblebeginning with a stop layer judiciously located in the buffer structure.An additional step to reform a buffer structure is preferably applied ifrequired to reform a substrate 1 equivalent to the substrate beforerecycling.

A recycling process that includes the removal of at least part theremainder of the original multi-layer structure I, to keep the entiresubstrate I may be used. A finishing step for the rest of themulti-layer structure I may be applied during recycling. The finishingstep may include, for example, CMP, heat treatment, sacrificialoxidation, bombardment, or other smoothing technique, or a selectivematerial removal may be applied using a stop layer judiciously locatedbetween the multi-layer structure I and the substrate 1.

After recycling of the substrate 1, a new multi-layer structure I isformed in which several useful layers may be taken off according to aprocess of the invention. This new multi-layer structure I may besimilar to the structure before recycling. This new multi-layerstructure I may have a slightly different structure from the multi-layerstructure I before recycling, by slightly modifying some manufacturingparameters. For example, the concentrations of some compounds in amaterial may be slightly modified. In all cases, the multi-layerstructure is preferably made by growth of layers, for example by epitaxyusing CVD or MBE.

In one embodiment, the growth of at least one layer within themulti-layer structure I is done in situ, directly in continuation withthe formation of the subjacent growth support, the subjacent growthsupport in this case also advantageously being formed by layer growth.In a second embodiment, at least one of these layers is grown after aslight finishing step on the subjacent growth support surface, such asby CMP polishing, heat treatment, or other smoothing technique.

In the remainder of this application, example configurations of donorwafers 10 are presented that include multi-layer structures I that canbe implemented by a process according to the invention. In particular,different materials will be presented that can advantageously be used insuch donor wafers 10. Some of the examples may include a bufferstructure and a support substrate in the substrate 1, the bufferstructure being formed on the support substrate. In some of theexamples, the buffer structure has a first lattice parameter at thelevel of its support substrate and a second lattice parameter close toits interface with the subjacent multi-layer structure I. This type ofbuffer structure includes a buffer layer to make such an adaptation ofthe lattice parameter.

The first technique for making a buffer structure (as described above),usually used to obtain a buffer layer with this property, is to have abuffer layer formed by several atomic elements, wherein at least oneatomic element is included in the composition of the support substrate,and at least one atomic element that is not located in the supportsubstrate at all, or only slightly, with a concentration that variesgradually through the thickness of the buffer layer. The gradualconcentration of this element in the buffer layer will be the main causeof the gradual variation of the lattice parameter in the buffer layer,metamorphically.

Thus, in this configuration, a buffer layer is mainly an alloy. Theatomic elements chosen for the composition of the support substrate forthe buffer structure and the buffer layer may be of atomic element GroupIV, such as Si or Ge.

For example, this embodiment includes a Si support substrate and a SiGebuffer layer with a concentration of Ge that varies gradually throughthe thickness between a value close to 0 at the interface with thesupport substrate and a determined value on the other face of the bufferlayer. In another case, the composition of the support substrate or thebuffer layer may include a pair of Group III-V atomic elements, such asa pair chosen from possible combinations of (Al, Ga, In), (N, P, As).

For example, one embodiment includes a support substrate of AsGa and abuffer layer of As and Ga and at least one other element, and varyinggradually in thickness between a value close to 0 at the interface withthe support substrate and a determined value on the other face of thebuffer layer. The composition of the support substrate or the bufferlayer may include a pair of Group II-VI atomic elements, such as a pairchosen among possible combinations of (Zn, Cd), (S, Se, Te).

EXAMPLES

The first five examples deal particularly with donor wafers 10 thatinclude a support substrate 1 made of Si and a buffer layer made of SiGeor Si and other layers of Si and SiGe within the multi-layer structureI. These wafers 10 are particularly useful when taking off layers ofstrained SiGe and/or Si to make SGOI, SOI or Si/SGOI structures. In thiscontext, the etching solutions used are different depending on thematerial (Si or SiGe) to be etched. Thus, etching solutions suitable foretching these materials will be classified in categories, assigning anidentifier taken from the following list to each category:

-   -   (A) S1: selective etching solutions of Si with respect to SiGe        such that a solution includes at least one compound among KOH,        NH₄OH (ammonium hydroxide), TMAH, EDP, or HNO₃, or solutions        combining agents such as HNO₃, HNO₂H₂O₂, HF, H₂SO₄, H₂SO₂,        CH₃COOH, H₂O₂, and H₂O as disclosed in WO 99/53539, page 9.    -   (B) S2: selective etching solutions of SiGe with respect to Si        such as a solution including HF, H₂O₂, CH₃COOH (selectivity        about 1:1000), or HNA (hydrofluoric-nitric-acetic solution).    -   (C) Sc1: selective etching solutions of SiGe with a        concentration of Ge significantly less than or equal to 20%,        with respect to SiGe with a concentration of Ge equal to or        greater than 25%, such as a solution containing TMAH or KOH.    -   (D) Sd1: selective etching solutions of undoped Si with respect        to boron doped Si, preferably doped to more than 2×10¹⁹ cm⁻³,        such as a solution containing EDP (ethylene diamine and        pyrocathechol), KOH, or N₂H₂ (hydrazine).

Example 1

The donor wafer 10 is formed of a substrate 1 formed of a supportsubstrate made of Si; a buffer structure formed of SiGe made accordingto the first technique for making a buffer structure, and including abuffer layer and an additional layer; and a multi-layer structure I thatincludes SiGe. The concentration of Ge in the buffer layer preferablyincreases progressively from the interface with the support substrate,to vary the SiGe lattice parameter as explained above. The thickness istypically about 1 to 3 micrometers for surface concentrations of Ge ofless than 30% to obtain a good structural relaxation at the surface, andto confine defects related to the difference in the lattice parametersuch that they are buried.

The additional layer is made of SiGe significantly relaxed by the bufferlayer, with an advantageously uniform concentration of Ge almostidentical to the concentration of the buffer layer close to theirinterface.

The concentration of germanium in the silicon within the relaxed SiGelayer is typically about 15% to 30%. This upper limit of 30% representsa typical limitation of current techniques, but it may vary in futureyears. The thickness of the additional layer may vary widely dependingon the case, with a typical thickness of about 0.5 to 1 micron.

As shown in FIG. 2 a, the multi-layer structure I before taking offpreferably includes a triplet of the following layers: layer 3A made ofsubstantially relaxed SiGe with a thickness greater than the thicknessof a useful layer to be taken off; a layer 2 on layer 3A; and a layer 3Bon the substantially relaxed SiGe layer 2 and with a thickness greaterthan the thickness of a useful layer to be taken off.

Layer 2 is formed of one of strained Si or strained SiGe. Where thelayer 2 is formed of strained Si or SiGe, the thickness of this layer 2must not exceed a critical thickness. For a layer 2 made of strained Siinserted between two layers of SiGe with a concentration of Ge of about20%, the critical thickness is typically of the order of about 20nanometers.

Several types of etching may preferably be used after taking off part oflayer 3B, depending on the material in layer 2. If the layer 2 is madeof strained Si, the overlying part made of SiGe is etched selectivelywith an S2 type solution, or after the layer 3B remaining after takingoff has been removed, the layer 2 is etched selectively with an S1 typesolution. If layer 2 is made of strained SiGe with a concentration of Geat least about 25% and the concentration of Ge in the overlying layer isup to about 20%, the overlying part made of SiGe is selectively etchedwith an Sc 1 type solution. If layer 2 is made of SiGe with aconcentration of Ge up to about 20% and the concentration of Ge in thesubjacent layer is at least about 25%, after the layer 3B remainingafter taking off has been removed, the layer 2 is etched selectivelywith a Sc1 type solution.

A layer 2 formed of SiGe or Si may also be doped with doping elementssuch as boron or phosphorus, in order to improve the selectivity ofchemical etching. In one particular configuration of the multi-layerstructure I, the multi-layer structure I includes several triplets ofthese layers 3A, 2 and 3B. In one particular embodiment, the multi-layerstructure I includes only pairs of layers 2 and 3, as shown in FIG. 3.Preferably, a layer 2 separating two consecutive triplets, such as thedonor wafer 10 shown in FIG. 4 will be present.

It is then preferable and convenient to find all taking off formulasinvolving one or several layers taken off in one or in severaloperations, separated by recycling processes according to the inventionand preferably including selective etching between SiGe and the materialin a layer 2.

Example 2

The donor wafer 10 is formed of a substrate 1 formed of a supportsubstrate formed of Si; a buffer structure formed of SiGe made accordingto the first technique for making a buffer structure, and including abuffer layer formed of SiGe and an additional layer formed of Ge; and amulti-layer structure I including one or more of AsGa and AlGaAs. Theconcentration of Ge in the buffer layer preferably increasesprogressively from the interface with the support substrate, to vary thelattice parameter of the Si support substrate with respect to thelattice parameter of the additional layer made of Ge. The concentrationof Ge in the buffer layer is increased from about 0 to about 100% orpreferably about 98% for a complete match in the theoretical meshbetween the two materials.

For example, as shown in FIG. 1 a, the multi-layer structure I beforetaking off preferably includes the pair of layers: a layer 2 formed ofAlGaAs; and a layer 3 on layer 2, layer 3 formed of substantiallyrelaxed AsGa and with a thickness greater than the thickness of a usefullayer to be taken off.

Taking off applies to the part of the multi-layer structure I abovelayer 2, and recycling includes selective chemical etching of layer 3with a selective etching solution, such as a solution containing citricacid (C₆H₈O₇) and oxygenated water with a pH of about 6 to 7 (theselectivity coefficient is typically 20), so that almost the entirelayer 3 can be taken off, in this case layer 2 behaving like an etchingstop layer.

In one particular configuration of the multi-layer structure I, themulti-layer structure I includes another layer made of AsGa subjacent tolayer 2. Taking off then applies to the part of the multi-layerstructure I above this other AsGa layer, and recycling includesselective chemical etching of layer 2 with a selective etching solution,such as a solution that includes dilute hydrofluoric acid (between about9% and 48%) (the selectivity coefficient is typically about 350 to10000), so that almost the entire layer 2 can be removed, with the othersubjacent AsGa layer behaving like an etching stop layer. In one case,the two selective etchings can be done one after the other in order toremove at least part of layer 3 and then to remove layer 2.

In one embodiment, the multi-layer structure includes several pairs ofthese layers 2 and 3. The multi-layer structure I is formed of pairs oflayers 2 and 3, as shown in FIG. 3.

It is then preferable and convenient to find all taking off formulasinvolving one or several layers taken off in one or in severaloperations, separated by recycling processes according to the invention,preferably including selective etching between AlGaAs and GaAs.

Example 3

The donor wafer 10 is formed of a substrate 1 formed of Si and amulti-layer structure I that includes Si before taking-off. As shown inFIG. 2 a, the multi-layer structure I before taking-off preferablyincludes a triplet of the following layers: a layer 3A made ofsubstantially relaxed Si with a thickness greater than the thickness ofa useful layer to be taken off; a layer 2 on layer 3A; and a layer 3B onthe substantially relaxed Si layer 2 and with a thickness greater thanthe thickness of a useful layer to be taken off. Layer 2 is formed ofone doped Si or strained SiGe. Where layer 2 is formed of strained SiGe,the thickness of this layer 2 must not exceed a critical thicknessrelated to the Ge concentration.

Several types of etching may preferably be used after taking off part oflayer 3B, depending on the material in layer 2. If the layer 2 is formedof doped Si, the overlying part formed of Si and left after taking offis selectively etched with Sd1 type solution. If the layer 2 is formedof strained SiGe, the overlying part formed of Si is etched selectivelywith SI type solution, or after the layer 3B remaining after taking offhas been removed, the layer 2 is selectively etched with an S2 typesolution.

Example 4

The donor wafer 10 is formed of a substrate 1 formed of a supportsubstrate made of Si and a buffer layer made using the second particulartechnique for making a buffer structure discussed above and in WO00/15885.

A first layer formed of Ge or SiGe is deposited, possibly followed bydeposition of a second optional layer that can improve thecrystallographic quality of the overlying layer, as disclosed in WO00/15885. The second layer is formed of SiGe (50/50), if the first layeris Ge; strained Si if the first layer in the buffer layer is made ofSiGe.

The donor wafer also includes a multi-layer structure I that includes asequence of pairs of layers, each pair formed of a relaxed layer 3 and astrained layer 2, each relaxed layer 3 is at least as thick as the auseful layer to be taken off and is formed of Ge, if the first layer ofthe buffer layer is formed of Ge, or SiGe if the first layer the bufferlayer is formed of SiGe (with a concentration almost the same as theconcentration of the first layer in the buffer layer). Each strainedlayer 2 is formed of strained Si or SiGe and its thickness is less thana critical thickness beyond which the elastic strain starts to relax,this critical thickness depending on the composition of the threeadjacent relaxed layers made of Si. Removal of the multi-layer structureI may apply to a set of layers or a single layer of the multi-layerstructure 1. Thus, a relaxed layer 3, a strained layer 2 or a set ofstrained layers 2 and relaxed layers 3 may be taken off, to transferthem onto a receiving substrate 5.

If taking off takes place in a relaxed layer 3, recycling according tothe invention can be implemented by chemically etching the remainingrelaxed layer 3 with a solution capable of etching the material in thelayer 3 selectively with respect to the material in layer 2. If layer 2is formed of Si and layer 3 is made of SiGe, the strained layer 2 willbe etched with S2 type solution, the strained layer 2 then being anetching stop layer. This etching may be followed by a second etching ofthe strained layer 2 that is selective with respect to another relaxedlayer. It is thus possible to implement a second taking off after afirst taking off, the second taking off applying to the strained layer 2or part of the subjacent multi-layer structure I.

Note that the structures obtained after taking off according to thisexample are free of any dislocation type defects, even in a buriedregion. The structures obtained as a result may then be used to growadditional layers, for example, formed of strained silicon, by epitaxyon the layer made of strained SiGe, Ge or Si.

Example 5

The donor wafer 10 is formed of a substrate 1 formed of a supportsubstrate formed of Si; a buffer layer formed of Si according to thethird technique for making a buffer structure; and a multi-layerstructure I that includes a triplet of the following layers: (1) a firstlayer 3A formed of SiGe with at least 15% of Ge on the buffer structure,the SiGe being relaxed or pseudo-relaxed; (2) a second layer 2 formed ofstrained Si with a thickness much less than the cumulative thickness ofthe first layer 3A and the second layer 3B; and (3) a third layer 3Bformed of SiGe and including at least 15% of Ge, the SiGe being relaxedor pseudo-relaxed. This donor wafer 10 is obtained after making thebuffer layer according to the third technique for producing the bufferstructure.

In a first embodiment of the buffer layer, the triplet of layers waspresent before the buffer layer was made. The donor wafer 10 is in theform of a substrate 1 formed Si; and a multi-layer structure I′ formedof the following layers: (1) a first layer 3A′ formed of SiGe with atleast 15% of Ge on the buffer structure, the SiGe being strained; (2) asecond layer 2′ formed of relaxed Si; and (3) a third layer 3B′ formedof SiGe that includes at least 15% of Ge, the SiGe being strained. Thestrained SiGe layer 3A′ has nearly the same characteristics as thestrained SiGe layer 3B′.

The density of defects such as dislocations in the multi-layer structureI′ is preferably less than about 107 cm⁻². Typical thicknesses of amulti-layer structure I′ with layers 3A′ and 3B′ with 15% of Ge and amulti-layer structure I′ with layers 3A′ and 3B′ with 30% Ge, are about250 m and about 100 nm respectively, thus remaining below the criticalend of elastic strain thickness for each layer. The typical thickness ofthe relaxed layer 2′ is a few tens of nanometers. The orders ofmagnitude of the thicknesses of the strained layers 3A′ and 3B′ arepreferably nearly the same as each other. Therefore, the multi-layerstructure I′ is globally strained.

The buffer layer will be made by the following steps: formation of adisturbance area in the support substrate 1 formed of Si by theimplantation of atomic species such as H or He; and heat treatment tocause at least relative relaxation of elastic strains in the multi-layerstructure I′.

Implantation energy ranges of H and He used during the first step aretypically about 12 to 25 keV. Implanted H or He doses are typicallyabout 10¹⁴ to 10¹⁷ cm⁻². Thus for example, for a strained layer 3A′ with15% of Ge, H will preferably be used for the implant at a dose of about3×10¹⁶ cm⁻² with an energy about 25 keV. For a strained layer with 2% to30% of Ge, He will preferably be used for the implant at a dose of about2×10¹⁶ cm⁻² with an energy about 18 keV. Implant depths of atomicspecies in the substrate 1 are also typically about 50 nm to 100 nm.

The heat treatment applied during the second step must be adapted sothat disturbances are displaced in the region between the disturbancezone and the multi-layer structure I′. This region in which disturbancesare displaced will then form the buffer layer.

The arrival of dislocations at the interface between the buffer layerand the multi-layer structure I′ causes a global relaxation of themulti-layer structure I′ as follows: elastic relaxation of the strainedlayer 3A′ to form the relaxed or pseudo-relaxed layer 3A; elastic strainin the relaxed layer 2′ to form the strained layer 2, this layer havinga lattice parameter nearly the same as the lattice parameter of thesubjacent relaxed SiGe; and elastic relaxation of the strained layer 3B′to form the relaxed or pseudo-relaxed layer 3B. The movement ofdislocations in the buffer layer also causes a large disappearance ofdislocations in the multi-layer structure I′.

The heat treatment is preferably implemented under an inert atmosphere.However, the heat treatment may be applied under another atmosphere,such as an oxidizing atmosphere. A particular heat treatment to beapplied for this type of donor wafer 10 will be made at temperatures oftypically about 400° C. to 1000° C. for about 30 s to 60 minutes,preferably about 5 minutes to about 15 minutes.

In a second embodiment of the buffer layer, the triplet of layers werenot present before the buffer layer was made. The donor wafer preferablyincludes a Si substrate 1 and a layer formed of SiGe including at least15% of Ge, the SiGe being elastically strained. Relaxation techniquesand parameters for this SiGe layer are nearly the same as for the firstembodiment of the buffer layer.

The next step after the buffer layer has been made is to grow layersforming the multi-layer structure that includes the triplet of globallyrelaxed layers. The multi-layer structure I is thus made after thebuffer layer, unlike in the first embodiment proposed in this example.For further information about experimental techniques, refer to studiesdone by B. Holländer et al., “Strain relaxation of pseudomorphicSi_(1-x)Ge_(x)/Si(100) heterostructures after hydrogen or helium ionimplantation for virtual substrate fabrication” (in Nuclear andInstruments and methods in Physics Research B 175-177 (2001) 357-367).

After the wafer 10 has been bonded onto a receiving substrate 5 on therelaxed layer 3B, taking-off is performed with or without anintermediate bonding layer using one or more of the known techniquesdescribed above. Some of the relaxed SiGe layer 3B is taken off.Recycling is preferably done by selective chemical etching of theresidue of layer 3B using a type S2 solution, layer 2 then forming anetching stop layer. A second selective chemical etching step of layer 2can then be applied using S1 type solution, the layer 3A forming anetching stop layer. The result is a recycled donor wafer 10 that cangive a new useful layer in layer 3A or a pair of layers 2/3A, by takingoff.

Example 6

The donor wafer 10 is formed of a substrate 1 formed of a supportstructure at least part of which is formed of AsGa at its interface withthe overlying buffer structure, a buffer structure formed of Group III-Vmaterial made according to the first buffer structure manufacturingtechnique, and a multi-layer structure I formed of a Group III-Vmaterial before taking-off. The support structure may be formed of solidAsGa or solid Ge, on which a thickness of AsGa is epitaxially grown.

The first advantage of the buffer structure is to adapt the latticeparameter with the material of the multi-layer structure I close totheir interface (which for example may have a nominal value of 5.87Angstroms in the case of InP), to the value of the AsGa (for which thenominal value is about 5.65 Angstroms).

In solid Group III-V materials, the practical advantage of such a bufferstructure may become clear by comparing different materials such assolid InP and solid AsGa. For example solid AsGa is less expensive, moreeasily available on the semiconductors market, less weak mechanically,with the use of the best known rear face contact technologies, andavailable in larger sizes (typically 6 inches instead of 4 inches forsolid InP) than for solid InP.

The electronic performances of InP, however, are usually more usefulthan the electronic performances of AsGa. Thus, the donor wafer 10 givesa solution for the manufacture of a 6-inch InP layer, by proposing amulti-layer structure I formed of InP on an AsGa support substrate andrelaxed through a buffer structure.

Therefore, the possible advantages of such a donor wafer 10 becomeevident—it can be used to make an active layer made of a known GroupIII-V material to be transferred with determined quality and properties,for example that may be similar to the properties that could have beenobtained if solid Group III-V material had been used.

The thickness of the buffer structure included within this type of donorwafer 10 is typically more than one micron, and this thickness willincrease, particularly if it is possible to avoid destroying it aftereach taking off operation due to the use of a recycling processaccording to this invention.

In the example of the multi-layer structure I formed of InP essentiallyrelaxed at its interface with the subjacent buffer structure, the bufferstructure of the substrate 1 then preferably includes a buffer layerformed of InGaAs with a concentration of In of about 0 to about 53%. Thebuffer structure may also include an additional layer made of a GroupIII-V material such as InGaAs or InAlAs, with an almost constantconcentration of atomic elements.

In one embodiment, at least one InP layer is taken off in themulti-layer structure I so that it can be transferred onto a receivingsubstrate 5. Thus, any electrical and electronic properties can be wellused. For example, this is the case if the taken off part also includesInGaAs or InAlAs; discontinuities in electronic bands between the lattermaterial and the InP create significantly better electronic mobilitiesin the taking off layers. Other configurations of donor wafers 10 arepossible, including other Group III-V compounds. Typical applicationsfor these means of taking off layers include HEMT (High ElectronMobility Transistor) and HBT (Heterojunction Bipolar Transistor)implementations. Chemical etching solutions, possibly selective andadapted to remove some Group III-V materials and not other Group III-Vmaterials, are preferably used during recycling. Thus, for example,selective etching of InP is preferably used with a solution formed ofconcentrated HCl in order to remove an InP layer without taking off asubjacent layer of InGaAs.

Example 7

The donor wafer 10 is formed of a substrate 1 formed of a supportsubstrate formed of AsGa at its interface with the overlying bufferstructure; a buffer structure made according to the first technique formaking a buffer structure, and including InGaAs at its interface withthe multi-layer structure I; and a multi-layer structure I formed of InPor In_(x)Ga_(1-x)As_(y)P_(1-y) before taking-off.

With reference to FIG. 1 a, the multi-layer structure I before takingoff Preferably includes the pair of the following layers: a layer 2 madeof InGaAs (P) and a layer 3 on layer 2, layer 3 being formed ofsubstantially relaxed InP and with a thickness greater than thethickness of a useful layer to be taken off. This type of donor wafer 10was described in example 6 above.

Taking off applies to the part of the multi-layer structure I abovelayer 2, and recycling includes selective chemical etching of layer 3with a selective etching solution, such as a solution that includes HFso that almost the entire layer 3 left after taking-off can be removed,in this case layer 2 behaving like an etching stop layer.

In one embodiment, the multi-layer structure I includes another layer ofInP subjacent to layer 2. Taking off then applies to the part of themulti-layer structure I above this other InP layer, and recyclingincludes selective chemical etching of layer 2 with a selective etchingsolution, such as a solution that includes Celv H₂SO₄, so that almostthe entire layer 2 is removed, in this case the other subjacent InPlayer behaving like an etching stop layer.

In another case, the two selective etchings can be done one after theother in order to remove at least part of layer 3 and to remove layer 2.In one particular configuration of the multi-layer structure I, themulti-layer structure I includes several pairs of these layers 2 and 3.The multi-layer structure I may include only pairs of layers 2 and 3,for example as shown in FIG. 3. It is then preferably and convenient tofind all taking off formulas involving one or several layers taken offin one or in several operations, separated by recycling processesaccording to the invention preferably including selective etchingbetween InP and InGaAs (P).

Example 8

The donor wafer 10 is formed of a substrate 1 formed of a supportsubstrate formed of sapphire, SiC, or Si, a buffer structure madeaccording to the first technique for making a buffer layer, and amulti-layer structure I formed of nitride layers. The buffer structureis formed of a metamorphic buffer layer of Al_(x)Ga_(1-x)N, where xvaries in thickness from 0 to 1 starting from the interface withsapphire, and an additional layer of GaN to confine dislocation typecrystallographic defects.

The Group III-V GaN, AIN and InN nitrides are useful inmicroelectronics, particularly in light emitting devices such as lasers,for applications such as reading or writing data stored at high densityon compact disks or such as light emitting diodes for new displaytechnologies. These materials are also suitable for making high powerelectronic components, or electronic components operating at hightemperature.

One method of making the nitride layers included in the multi-layerstructure I is epitaxial growth on the additional layer of GaN bydeposition of Group I organic metals, such as Trimethyl Gallium,Trimethylamine Alane, or Trimethyl Indium for the deposition of the GaN,AIN, or InN layers, respectively. This invention used to transferseveral of these nitride layers starting from the same donor wafer 10implies a recycling step between each operation to take off a layer, inorder to prepare another layer in the multi-layer structure I foranother taking off operation. Thus, several techniques, mainly smoothingof layers by chemical etching, help to achieve this purpose whilekeeping the structural and geometric quality of the layer to be takenoff intact or almost intact. One example is a photo etching techniqueused to etch layers of GaN, as disclosed in R. T. Leonard et al.,“Photo-assisted dry etching of GaN”, in Applied Physics letters 68 (6),Feb. 5, 1996.

In the present example, the multi-layer structure I including nitridesis similar to that shown in FIGS. 2 and 4. As shown in FIG. 2 a, themulti-layer structure I before taking off preferably includes a tripletformed of a layer 3A of AIN, a layer 2 of InN, and a layer 3B of GaN.Preferably, the multi-layer structure I includes another almostidentical triplet of layers, separated from the first triplet by a layerof InN.

The advantage of this three-layer structure lies in the choice ofmaterials for which chemical attack means are available that aresignificantly different for each of the materials. Thus, InN has a lowerdry etching rate than GaN or AlN if a plasma gas that includes polarizedchlorine, hydrogen, and possibly argon is injected on the wafer 10,particularly adapting the technical parameters as follows: power ofpolarizing radio frequencies preferably about 400 to 1000 Watts, morepreferably about 650 Watts; temperature of about 500° to 1000°K,preferably closer to 1000°; low pressure on the order of 1 mT; andcomposition with Cl₂ to H₂ ratio on the order of 2 to 3, for a totalfluidic ratio of about 25 sccm.

The etching selectivity of GaN and AlN compared with InN is mainly dueto the low volatility of InCl_(x) species compared with the volatilityof species containing Ga and Al. N atoms in nitrides combine very wellwith H₂ to form gaseous molecules of NH₃. With reference to theexperimental results obtained by S. J. Pearton et al. in “Law BiasElectron Cyclotron Resonance Plasma Etching of GaN, AlN and InN”(Applied Physics Letters 64 (17), Apr. 25, 1994), the etching ratio ofGaN with respect to InN may be more than 3 to 1, and the etching ratioof AIN with respect to InN may be of the order of 5 to 1. An etchingratio of GaN with respect to AIN on the order of 2 to 1 can be obtained,and a dry etching ratio of InN with respect to AlN of the order on 3 to2 can be obtained.

If taking off is done in the GaN layer, a gas including polarizedchlorine may preferably be used so that the subjacent layer of InN formsan etching stop layer. If it is not required to keep the InN layer, dryetching can be applied using a gas containing CH₂, so that the subjacentlayer of AIN forms an etching stop layer.

An additional finishing step, for example polishing, may be used tofinish the surface of the layer kept after etching. This layer keptafter etching may then be taken off again. Similarly, if taking off isdone in the InN layer, the residue of the InN layer can advantageouslybe dry etched using gas containing CH₂ so that the InN layer forms astop layer. If it is not required to keep the layer made of AlN, dryetching using gas including chlorine may be used so that the subjacentInN layer forms a stop layer. Finally, if a layer made of AlN is takenoff, dry etching can advantageously be used using chlorine gas while thesubjacent InN layer forms a stop layer. Several layers may also be takenoff in a single taking off operation, particularly if there are severaltriplets (AIN, InN, GaN) separated by an InN layer.

Example 9

The donor wafer 10 is formed of a substrate 1 formed of a supportsubstrate formed of sapphire or SiC or Si; an intermediate layer formedof GaN; a SiO₂ mask; a GaN buffer layer; and a multi-layer structure Ithat includes successive layers of nitrides, including at least onelayer of GaN.

The method of making the buffer layer is as described above for thefourth technique for making the buffer structure, and includesanisotropically growing nitride layers, particularly GaN, using the ELOGtechnique. The SiO₂ mask used in this configuration is preferably in theform of bands arranged periodically on the intermediate layer of GaN,and nearly parallel to each other. The thickness of each band istypically of the order of a few tens of a micron, while the width of aband is of the order of a few microns. The distance separating the bandsfrom each other is typically about 10 microns or 15 microns. Forexample, there could be a network of bands at 13 micron intervals, eachband being 0.2 microns thick and 5 microns wide.

As described above in a general case, these SiO₂ bands will causelocalized dislocations in the GaN layer(s) deposited above, close to thefree surface of these bands. The thickness of GaN in which thesedislocations are located around the mask then forms the buffer layer.The layers of GaN or other materials with lattice parameters similar tothe lattice parameters of GaN are deposited on the buffer layer to formthe multi-layer structure I. The multi-layer structure I includes atleast two layers each with a thickness equal to or greater than thethickness of a useful layer that is to be taken off. Further informationabout the method of making a wafer according to the ELOG process isdisclosed in “MRS Bulletin” May 1998, volume 23, No. 5, Article by ShujiNakamura entitled “In/GaN/AlGaN-Based Laser Diodes with an EstimatedLifetime of Longer than 10,000 Hours”.

In particular, in this multi-layer structure I, stop layers withselective chemical etchings such as InN layers as described in example 8may be integrated during manufacturing. Thus, after taking off a layerof GaN, recycling according to the invention may be include selectivelyetching the GaN with respect to the subjacent InN layer using an etchinggas containing polarized chlorine as described in example 8. Otherconstituents may be added into the semiconductor layers, such as carbonwith a carbon concentration in the layer significantly less than orequal to about 50%, preferably less than or equal to about 5%.

Finally, this invention is not restricted to a donor wafer 10 made ofmaterials presented in the above examples, but also covers other typesof materials belonging to Groups II, III, IV, V or VI and to alloysbelonging to Groups IV-IV, III-V, II-VI. In the case of alloy materials,the alloys chosen may be binary, ternary, quaternary, or higher degree.If the donor wafer 10 includes a buffer layer or a buffer structure,this invention is not limited to a buffer layer or a buffer structurewhich has the prime function of adapting the lattice parameter betweentwo adjacent structures with different lattice parameters, but alsoconcerns any buffer layer or buffer structure as defined more generallyin this document. Furthermore, the final structures obtained as a resultof taking off useful layers are not restricted to SGOI, SOI, Si/SGOIstructures, nor are they restricted to structures for HEMT and HBTtransistors nor to structures for applications in lasers.

It is to be understood that the invention is not to be limited to theexact configuration as illustrated and described herein. Accordingly,all expedient modifications readily attainable by one of ordinary skillin the art from the disclosure set forth herein, or by routineexperimentation therefrom, are deemed to be within the spirit and scopeof the invention as defined by the appended claims.

1. A method of transferring useful layers from a donor wafer whichcomprises: forming a multi-layer structure on a surface of the donorwafer at a thickness sufficient to provide multiple useful layers forsubsequent detachment; detaching one layer of the multi-layer structurefor transfer as a first useful layer while leaving behind a remainingportion of the multi-layer structure; and removing material from theremaining portion of the formed multi-layer structure to provide aplanarized surface for subsequent detachment of an additional usefullayer.
 2. The method of claim 1, wherein the multi-layer structure hassufficient thickness to form at least three useful layers.
 3. The methodof claim 1, wherein the useful layers are formed of materials havingsufficiently different properties such that one layer may be selectivelyremoved while at least one other layer remains intact.
 4. The method ofclaim 1, wherein the useful layers include pairs of sub-layers, witheach sub-layer having sufficiently different properties from the othersub-layer in the layer such that one sub-layer may be selectivelyremoved while at least one other sub-layer remains intact.
 5. The methodof claim 1, wherein a first layer of the multi-layer structure is formedof a material having a lattice parameter different from the latticeparameter of the adjacent layers, and is sufficiently thin such that thefirst layer may be elastically strained to have a lattice parametersimilar to the lattice parameter of the adjacent layers, so that thefirst layer does not disturb the crystallographic structure of theadjacent layer or layers.
 6. The method of claim 1, wherein at least onelayer includes a doping element to facilitate selective removal, or thelayers are made of materials having different porosities.
 7. The methodof claim 6, wherein the selective removal of material comprisesdeoxidation of an oxide layer.
 8. The method of claim 1, wherein atleast one useful layer is removed by chemical etching.
 9. The method ofclaim 1, wherein at least one useful layer is removed by electrochemicalor photo-electrochemical etching.
 10. The method of claim 1, wherein atleast one additional useful layer is removed mechanically by a selectivemechanical-chemical planarization.
 11. The method of claim 1, whereinall useful layers are removed mechanically.
 12. The method of claim 11,wherein the layers are removed by polishing, optionally includingabrasion or chemical etching.
 13. The method of claim 1, which furthercomprises forming a buffer structure on the donor wafer, wherein thebuffer structure includes a support substrate and a buffer layer betweenthe support substrate and the multi-layer structure.
 14. The method ofclaim 13, wherein the buffer layer comprises a semiconductor material ofconstant chemical composition having a lattice misfit with the supportsubstrate, with the buffer layer having crystallographic defects inorder to relax elastic stresses in the multi-layer structure.
 15. Themethod of claim 13, wherein the buffer layer is formed of one or more ofSi, SiGe, Ge or a nitride material.
 16. The method of claim 13, whereinthe multi-layer structure comprises one or more of: elastically strainedSi, SiGe or Ge.
 17. The method of claim 13, wherein the buffer layer hasa lattice parameter progressing substantially in thickness between alattice parameter of the support substrate and another lattice parametersubstantially different from that of the support substrate.
 18. Themethod of claim 13, wherein the buffer structure further comprises anadditional layer having one or more of a sufficient thickness to confinedefects or a surface lattice parameter that is different from that ofthe support substrate.
 19. The method of claim 13, wherein the bufferstructure is formed of a binary, ternary, quaternary, or higher degreeatomic alloy belonging to atomic alloy Group IV-V, Group III-V, or GroupII-VI.
 20. The method of claim 13, wherein the donor wafer comprises oneof the following: (A) a support substrate of Si and a buffer structurecomprising a buffer layer of SiGe having a Ge concentration whichincreases in thickness and an additional layer of SiGe that is relaxedby the buffer layer; (B) a support substrate of Si and a bufferstructure comprising a buffer layer of SiGe having with a Geconcentration which increases in thickness between about 0% and about100% and an additional layer of SiGe that is relaxed by the bufferlayer, having a Si concentration of about 0%; (C) a relatively thicklayer of Si; (D) a support substrate comprising AsGa in contact with thebuffer structure, the buffer structure comprising a buffer layer havingan atomic alloy of ternary or higher degree, belonging to Group III-V,and having a composition which chosen from among the possiblecombinations of (Al, Ga, In), (N, P, As), and at least two elementschosen from Group III or V, these two latter elements having aconcentration evolving gradually in the thickness of the buffer layer;(E) the buffer structure of (D) which further comprises, near itsinterface with the support substrate, a lattice parameter that is closeto that of InP; (F) a support substrate of sapphire, SiC, or Si, with abuffer structure comprising a buffer layer of Al_(x)Ga_(1-x)N, with xvarying from 0 to 1 starting from the interface with the supportsubstrate, optionally including an additional layer of GaN; or (G) asupport substrate of sapphire, SiC, or Si, a mask and a buffer layer ofGaN, optionally with another layer of GaN between the mask and supportsubstrate.
 21. The method of claim 20, wherein the useful layers to bedetached comprise Si or elastically stressed Si; Ge or elasticallystressed Gei, SiGe and Ge; AsGa and Ge; an alloy belonging to GroupIII-V; GaAs, InP; InGaAs; AlN, InN, or GaN.
 22. The method of claim 1,wherein the multi-layer structure comprises one of the following: (A)two layers of almost elastically relaxed SiGe, and an intermediate layerbetween the SiGe layers comprising: Si elastically strained to have alattice parameter similar to the lattice parameter of the adjacent SiGelayers; SiGe with a Ge concentration substantially different from the Geconcentrations in each of the two adjacent layers, and being elasticallystrained to have a lattice parameter similar to the lattice parameter ofthe adjacent layers; doped Si; or doped SiGe; wherein at least one ofthe layers has properties sufficiently different from the other layersto permit selective chemical etching of the adjacent layer; (B) twolayers of GaAs, and an intermediate layer of AlGaAs intermediate betweenthe GaAs layers, wherein at least one of the layers has propertiessufficiently different from the other layers to permit selectivechemical etching of the adjacent layer; (C) two layers of substantiallyelastically relaxed Si, and an intermediate layer between the Si layerscomprising SiGe elastically strained to have a lattice parameter similarto the lattice parameter of the adjacent Si layers, doped Si, or dopedSiGe; wherein at least one of the layers has properties sufficientlydifferent from the other layers to permit selective chemical etching ofthe adjacent layer; (D) two layers of InP, and an intermediate layer ofInGaAsP between the two layers of InP, wherein at least one of thelayers has properties sufficiently different from the other layers topermit selective chemical etching of the adjacent layer; (E) anintermediate layer of InN between a AlN layer and a GaN layer; or (F) anintermediate layer of InN between two layers of GaN.
 23. The method ofclaim 1, which further comprises detaching at least a second usefullayer from the remaining portion of the multi-layer structure.
 24. Themethod of claim 13, wherein the buffer layer is formed by epitaxialgrowth of a superficial layer on the support substrate by: stabilizingthe support substrate to a predetermined first temperature; chemicalvapor phase depositing of a base layer at the said predetermined firsttemperature to a predetermined thickness that is less than a desiredfinal thickness; increasing the predetermined first temperature to asecond predetermined temperature; and chemical vapor phase depositingadditional material at the predetermined second temperature until thedesired final thickness of the superficial layer is obtained; whereinthe buffer layer is that portion of the superficial layer thatinterfaces with the support substrate and extends over a thickness forwhich the rate of crystallographic defects is greater than a limitvalue, with the remaining portion of the superficial layer representingbeing at least a portion of the multi-layer structure.
 25. The method ofclaim 24, wherein the first predetermined temperature is about 400° C.to 500° C., and the second predetermined temperature is about 750° C. to850° C.
 26. The method of claim 13, wherein the buffer layer is formedby: forming an elastically stressed layer on the donor wafer; forming aperturbation zone, capable of forming structural perturbations, at agiven depth; and supplying energy to bring about an at least relativerelaxation of at least part of the elastically stressed layer, with therelative relaxation taking place across a region delimited by theperturbation zone and by the stressed layer, this region also confiningcrystallographic defects and representing a buffer layer.
 27. The methodof claim 26, wherein the perturbation zone is formed by implantation ofatomic species.
 28. The method of claim 27, wherein the implanted atomicspecies comprise at least in part comprises hydrogen or helium.
 29. Themethod of claim 26, wherein the supplied energy is thermal energy. 30.The method of claim 13, wherein the buffer layer is formed by ELOG on asupport substrate comprising at the surface a mask in relief accordingto specific patterns, the buffer layer being the thickness of the layerdeposited by lateral epitaxy in which defects are confined relative tothe layers deposited by lateral epitaxy thereon and representing themulti-layer structure.
 31. The method of claim 30, wherein the specificpatterns of the mask are periodically spaced electrically insulatingbands that are substantially mutually parallel and sufficiently fine tonot perturb the ELOG.
 32. The method of claim 30, wherein the supportsubstrate comprises a solid substrate; an intermediate layer having alattice parameter close to the nominal lattice parameter of the materialdeposited by ELOG; or the mask.
 33. The method of claim 13, wherein thebuffer layer is formed so as to have its lattice parameter progressingsubstantially in thickness between the lattice parameter of the supportsubstrate and another lattice parameter substantially different from thelattice parameter of the support substrate.
 34. The method of claim 13,wherein the buffer structure further comprises an additional layer, andthe method further comprises, before the formation of the multi-layerstructure, forming of an additional layer having a sufficient thicknessto confine defects or a surface lattice parameter that is substantiallydifferent from that of the substrate.